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New optoelectronic LDPD speeds CMOS imaging

Posted: 06 Jan 2012     Print Version  Bookmark and Share

Keywords:image sensor  optoelectronics  photodetector  image processing 

Researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems have designed and patented a new CMOS image sensor solution for use in low light situations wherein conventional CMOS sensors are not suitable. The new optoelectronic component is called a lateral drift field photodetector (LDPD).

Werner Brockherde, head of department at the Fraunhofer Institute explains, "In this component, the charge carriers generated by the incident light move at high speed to the readout node."

In conventional CMOS image sensors that use pinned photodiodes (PPD) the electrons simply diffuse to the exit in a comparatively slow process that is sufficient for many applications. "But by integrating an internal electric field into the photoactive region of the component, we have managed to accelerate this process by a factor of up to a hundred," Brockherde says.

CMOS speed limit
Until now CMOS image sensors have been the solution of choice for digital photography. They are cheaper to produce than other sensors, and are better in terms of power consumption and handling. Thus, manufacturers of cell phones and digital cameras fit CMOS chips in their products to decrease power demand on the battery and make smaller cameras possible.

Yet conventional CMOS image sensors are reaching their limits: while miniaturisation in CE is leading to pixels 1µm across, some applications require pixels larger than 10µm, especially in areas where minimal light is available, such as in X-ray photography or in astronomy, having a larger pixel area compensates for the lack of light. Pinned photodiodes (PPD) are used to convert the light signals into electrical pulses. These optoelectric components are needed for image processing and are built into the CMOS chips. However, PPDs have a speed problem, which is now being solved by LDPD.

To produce the new component, the researchers improved the available CMOS chip manufacturing process based on the 0.35µm standard: "The additional LDPD component must not be allowed to impair the properties of the other components," says Brockherde. Using simulation calculations the experts managed to meet these requirements—and a prototype of the new high-speed CMOS image sensors is already available. "We expect to get approval for series production next year," says Brockherde.

The high-speed CMOS sensors are suitable for applications that need large pixels and a high readout speed, such as astronomy, spectroscopy and state-of-the-art X-ray photography. They are also suited for use as 3D sensors based on the time-of-flight process, whereby light sources emit short pulses that are reflected by the objects. The time-of-flight of the reflected light is then recorded by a sensor and used to create a fully-fledged 3D image. This technology is a compelling proposition for applications such as crash protection, as the sensors can precisely record their environment in three dimensions. The Fraunhofer researchers have already developed this kind of area sensor based on the unique pixel configuration for TriDiCam GmbH.

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